How does the Coriolis force twist fluid motion in Earth’s outer core?

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geomagnetic field The magnetic field of a bar magnet has a simple configuration known as a dipole field. Close to Earth's surface this field is a reasonable approximation of the actual field.

In the context of Earth’s geomagnetic dynamo, the Coriolis force is essential for twisting fluid motion to sustain the planet's magnetic field.1

As thermal convection drives fluid to rise within the outer core, the Coriolis force—which acts on fluids within a rotating body—deflects this rising motion. This deflection causes the fluid to spin around the central axis of the convection cell.1

This twisting action is critical to the "alpha effect": as the fluid spins, it twists the toroidal magnetic field carried by the rising fluid. After rotating approximately 270°, the magnetic field lines twist about themselves and diffuse through the conducting fluid, eventually reorienting into a poloidal field that reinforces the original magnetic field. Without this rotational twisting provided by the Coriolis force, the complex regeneration of the magnetic field would not occur.1